An underwater clearing device of the type described above is known from the German Patent Application 25 51 907. The round sewage treatment basin is spanned by a bridge arranged in a fixed position, which is supported on the crown of the basin. Assemblies consisting of two concentric hollow cylinders are suspended from the stationary bridge, carried by a central support- and guide-bearing. From the outer hollow cylinder jib booms radially extend, from which rake blades are suspended, which sweep along close to the bottom and are supported by support wheels. A motor with a driving pinion is supported on the stationary bridge, the driving pinion working together with a toothed ring on the outer cylinder. By way of the cylinder-shaped assemblies the actuation also of the rake blades is centrally effected. The rake blades may be formed to partially extend into the funnel-shaped sludge discharge.
It is an advantage of this known underwater clearing device that the crown of the basin is not stressed alternatingly and by a rotating bridge, but only through the fixed support of the stationary bridge. The flow through the hollow-cylindrical assemblies is improved. A disadvantage is the elaborate bridge with its support as well as a certain influencing of the flow in the round sewage treatment basin by the cylinder-shaped assemblies.
From the German Offenlegungsschrift 35 40 606 an underwater clearing device with a rotating bridge is known, which is supported on the crown of the basin. The rake blades are suspended from the bridge by struts in a radial inside region, so that the drive of the bridge simultaneously drives the rake blades. In this way the motor and the other aggregates, as for instance a transmission, are advantageously placed above and therefore out of the water. A central pillar is provided, on which the bridge is additionally supported. This type of underwater clearing device with a revolving bridge is very elaborate. It leads to damages of the crown of the basin, especially since it has to be large and heavy. For instance, the bridge has to able to withstand the pressure of the wind and nevertheless function properly. A disadvantage is the central pillar, which has to be built separately in the round sewage treatment basin. The drain for the waste water is provided inside the central pillar. The presence of the central pillar influences the flow in the round sewage treatment basin. Furthermore, the struts for the suspension of the rake blades from the bridge are necessarily led over the complete level of the water and influence the flow in the round sewage treatment basin negatively. A certain stirring action takes place, which has the consequence of a disadvantageous whirl-up. Because of the protruding struts, only a small part of the round sewage treatment basin can be used for the arrangement of packets of lamellas.